1. Field of the Invention
This invention relates to a process for multiple-coat reverse coating using at least two different polyurethane coating solutions, all the coating solutions containing one and the same polar solvent, preferably substantially anhydrous dimethyl formamide. By choosing
(A) surface-coat polyurethane solutions specially made up using high-melting diol chain-extending agents and by using PA0 (B) highly concentrated adhesive-coat polyurethane solutions containing at least 40% by weight of polyurethane, PA0 (A) the surface coats used are aromatic or aliphatic one-component or two-component polyurethanes which are synthesized using chain-extending agents based on low molecular weight diols melting at temperatures above 100.degree. C. and corresponding to formulae (I) to (VI) below ##STR1## in which R represents C.sub.2 -C.sub.20 and preferably C.sub.2 -C.sub.15 alkylene, cycloalkylene or arylene radicals, PA0 (B) the adhesive coats used are aromatic or aliphatic one-component polyurethanes different from (A), two-component polyurethanes and/or high-solids PU-reactive mixtures which have a solids concentration of more than about 40% by weight (preferably more than about 50% by weight) and PA0 (C) the solvent used for (A) and (B) is one and the same polar solvent selected from the group consisting of dimethyl formamide, dimethyl acetamide, N-methyl pyrrolidone, tetrahydrofuran and methyl ethyl ketone, preferably dimethyl formamide.
it is possible to effectively prevent the so-called "frostwork effect" when applying the adhesive coat to the surface coat (A) by the reverse coating process and the troublesome "overlamination" of the web-form textile during its lamination onto the PU-layers formed by reverse coating. In addition, recovery of the solvent from the coating solutions is simplified.
2. DESCRIPTION OF THE PRIOR ART
It has been known for some time that textiles (for example woven and knitted fabrics or nonwovens) may be coated with solutions of polyester and/or polyether urethanes by the reverse coating process. The articles obtained are used in the manufacture of outer clothing, upholstery, suitcase material, shoe upper materials and the like. The manufacture of shoe upper materials is of particular commercial significance.
A typical reverse coating process is carried out for example as follows:
In a coating machine, the surface coat solution (A) is doctor-coated onto a release paper, for example by means of doctor rolls, the spread amounting for example to 120 g/m.sup.2 of solution. After passage through a drying tunnel, which has an air temperature in the range from 80.degree. to 100.degree. C. at its entrance and in the range from 120.degree. to 140.degree. C. at its exit, the adhesive-coat solution (B) is similarly applied in a second coating machine, for example in a spread of 120 g/m.sup.2, after which the web-formed textile, for example a raised cotton-duvetine fabric weighing 240 g per square meter, is laminated onto the solvent-moist adhesive coat and the solvent mixture of the adhesive coat evaporated in the drying tunnel. On leaving the drying tunnel, the release paper is rolled up separately from the coated fabric web (artificial leather). This general version of reverse coating onto release paper can involve considerable difficulties which make it impossible to manufacture commercially satisfactory, coated textile webs.
The so-called "frostwork effect" can occur when the adhesive-coat solution (B), which may be both a one-component or two-component polyurethane and/or a high-solids polyurethane reactive mixture, is applied onto the dry, approximately 0.03 to 0.05 mm thick surface coat.
This figurative expression describes the following phenomenon. If the solvent mixture of the adhesive coat (B) is a poor solvent for the polyurethane of the surface coat (A), the surface coat (A) is not dissolved, but merely swollen. The effect of this swelling is that the surface coat film is lifted off the release paper at numerous points of the web, but still adheres to the web at other points. This swelling phenomenon, beginning at several points, spreads over the entire surface in a matter of seconds like freezing frostwork on a cold window and makes the coating unsuitable for further processing.
To avoid this effect, solvents which not only swell but also dissolve the surface coat have hitherto been used in the adhesive coat.
However, if the dissolving power of the adhesive coat solution is too great for the surface coat, the phenomenon of so-called "overlamination" is observed. The already dry surface coat is dissolved by the solvent (mixture) of the adhesive coat to such an extent that the web-form textile laminated onto the adhesive coat is forced through both polyurethane layers so that the textile structure, for example the pile, becomes visible on the upper side of the coating. This is inevitably accompanied by a marked deterioration in the physical strength values of the coatings, for example their resistance to bending and their resistance to abrasion.
Previous experience has shown that an adhesive coat solution always has an excessive dissolving power on the surface coat if it contains a high percentage of, or even exclusively, highly polar solvents, such as dimethyl formamide or dimethyl acetamide, in the solvent mixture.
In the field of pollution control, the burning or recovery of organic solvents from industrial processes, including for example the coating of textiles with polyurethane solutions, is becoming an acute industrial problem. The ecologically necessary recovery of solvents from textile coating processes can only be carried out rationally and economically when, contrary to the existing state of the art, the polyurethane solutions cohtain a single solvent rather than solvent mixtures, such as dimethyl formamide/methyl ethyl ketone, dimethyl formamide/methyl ethyl ketone/toluene, toluene/isopropanol and other mixtures. For reasons associated with the solubility of polyurethanes, the only solvents suitable for this purpose are highly polar solvents, such as dimethyl formamide, dimethyl acetamide, monomethyl formamide, monomethyl acetamide, N-methyl pyrrolidone and, in exceptional cases, less polar solvents, such as tetrahydrofuran or methyl ethyl ketone. In this connection, particular significance is attributed to dimethyl formamide by virtue of its dissolving power and its commercial availability.
Accordingly, there is a need to find a process where only one solvent, preferably dimethyl formamide, is used for coating without the coating process being accompanied by the adverse side effects mentioned above, such as the frostwork effect and overlamination.
Although it is known that, to reduce the effect of dimethyl formamide adhesive-coat solutions on surface coats already applied (dried on) to release paper, measured quantities of water may be added to the adhesive-coat solution (DE-OS No. 24 31 846), this reduces the solubility of the adhesive-coat polyurethanes in such a way that the solids concentration of the adhesive coat has to be kept relatively low.